§ 3.3.3 Electric Propulsion for Inter-Orbital Vehicles

Contents

• Overview
• Lunar and asteroidal propellants in electric propulsion
• How ion drive works (technical)
• How the Russian electric vehicle works (technical)
• MPD and other electric thrusters
• Mass Driver not considered seriously
• A problem for electric vehicles passing through medium Earth orbit
• Final words
• Links to other sites on electric propulsion

Overview

There are various ways of using electricity to thrust propellants, rather than using chemical explosion as in launch rockets.

Electric thrusting of propellants is useful only for interorbital transportation, not launch from the Earth's or Moon's surface.

There are two electric propulsion techniques used in space today:

• Some American interorbital satellites today use electric "ion drive" for stationkeeping. Ion drive is a simple and fairly mature technology. Ion drive can just as easily be used as the primary means of interorbital propulsion for delivering cargoes, and has been projected in future scenarios for space industrialization as a competitive kind of reusable interorbital vehicle. The Deep Space 1 probe, launched on October 24, 1998, was the first vehicle to depend upon electric ion drive for all of its propulsion needs, to perform a close flyby of an asteroid.
• The Russians have for at least 10 years been extensively using an electric propulsion technique called a "plasma thruster" which they have begun to market overseas. This thruster has been used on close to a hundred Russian military satellites, but is relatively unknown in the West.

The main advantages of electric propulsion are:

• It uses much less propellant than chemical rocketry (or, from another perspective, it gets much more mileage out of a given quantity of propellant)
• It could use 100% lunar or asteroid derived propellant
• It may promise better reliability and simplicity than chemical rocketry

Its main disadvantages are:

• It requires an electric power plant
• It offers only low thrust propulsion, which means a longer time to deliver the cargo

For an analogy, chemical rockets are like express delivery via powerful and fast airplanes, whereas ion drive vehicles in interorbital space are like the big tankers on the oceans which deliver their cargo slowly but cheaply and safely via surface transport and more mundane technology.

For missions to asteroids, it is actually advantageous to use a continuous, low thrust vehicle as this greatly expands the "launch window" period in which it is economically feasible to go to these objects, as compared to chemical rockets which impart short blasts of acceleration and deceleration. (This is also called a "thrust profile".)

Whereas chemical rocketry uses a chemical reaction and controlled explosion for thrust, electric propulsion uses electricity to accelerate the propellant out of the thrust chamber. Unlike chemical rocketry, there is no chance for an explosion with ion drive. Ion drive is much safer and a simpler technology, and has no relation to chemical rocketry at all.

Since electric propulsion vehicles use electricity, the vehicle must produce that electricity, typically by solar cell panels. In other words, ion drive vehicles are solar powered vehicles. The DS1 ion drive thruster is a 2.5 kilowatt device powered by a solar cell array.

Many studies into future large scale space industrialization base all interorbital propulsion on reusable electric propulsion vehicles, with chemical rockets being used only to launch material from Earth's surface to low Earth orbit, and from the lunar surface to lunar orbit, where the cargo is transferred to an electric propulsion vehicle for interorbital transport. However, some very conservative "immediate term" studies assume all interorbital propulsion is based on today's chemical rockets, e.g., with oxygen and hydrogen extracted from asteroids or the Moon being used to refill the fuel tanks of the chemical rockets, which means no use of electric propulsion. That's because lunar and asteroidal materials can be used to refuel today's hydrogen-oxygen chemical rockets with no design modifications at all, whereas large scale electric propulsion vehicles would need development of vehicle designs.

For a comparison between electric propulsion and chemical rocketry in the mainstream studies: Chemical rockets consume 8 TIMES as much fuel propellant than electric propulsion for the same service. The cargo-to-propellant ratio from low Earth orbit to high Earth orbit is around 4:1, round trip, with ion drive vehicles refueling in high orbit where lunar or asteroid derived propellants are tanked. In comparison, for today's chemical rocketry used for transporting satellites from low to high orbit, it's a 1:2 cargo to payload radio (not 2:1, but 1:2), and the vehicle goes on a one-way trip, being discarded afterward. Ion drive ejects its propellant at a speed about 15 times that of chemical rockets, hence imparting 15 times more momentum per unit mass of propellant. However, the ion drive vehicle is heavier than a chemical rocket vehicle due to the electric power plant so that the performance comes down to about 800% better than chemical rocketry. Yes, 800%.

(Notably, in a few studies, the propellant for ion drive vehicles is stated to be pure oxygen from the Moon or asteroidal material. This is a highly questionable assumption. Oxygen presents several problems for ion drive, as discussed later. However, other lunar and asteroid derived propellants should work fine. Also, oxygen should work in the Russian plasma thruster.)

One reason why only chemical rockets have been used in space for propulsion to date (except by the Russian military, addressed later) instead of ion drive is the continued lack of basic infrastructure in Earth orbit -- there is no reusable interorbital vehicle service in space at all. To date, the interorbital vehicle has always been launched with the payload and thrown away after it delivers its payload. To date, ion drive has been used only for stationkeeping propulsion once the satellite is delivered and its solar panels deployed. Hughes Space and Communications Company is starting to market an ion drive system as an interorbital upper stage, as covered at the end of this article. Nobody is yet marketing a reusable interorbital vehicle, whereby we just launch up fuel tanks and dock for fuel and payload transfer.

It is hoped that the DS1 mission will stimulate more interest in ion drive and electric propulsion in general, perhaps leading to a commercial venture to offer interorbital services via a reusable interorbital vehicle. These services could include hauling satellites from low orbit to geosynchronous orbit, moving old satellites to new orbits, and refueling and maintenance of satellites in orbit. Future satellites could be designed for interaction with such interorbital infrastructure, though this is a chicken-and-egg situation that must be overcome by broad industry recognition.

The Russian plasma thruster is used primarily on military satellites (e.g., space based radars), and their commercial space program has struggled to get onto its feet after the massive Cold War subsidation of their military program waned. The Russian plasma thruster is a well developed and very efficient engine, and is seen as one of the potentially most valuable exports in their space program. The Russian plasma thruster is seen by many in the Western space program as an unusual technology, but its efficiency and reliability in space for more than 10 years on nearly 100 military satellites makes it a very well proven piece of flight hardware.

DS1 is a step in the right direction to gaining acceptance of electric propulsion in the west, by using ion drive to get from Earth to an asteroid. A big part of the DS1 mission is simply to demonstrate and analyze the ion drive propulsion system in space. DS1 will carry a set of sensitive instruments to analyze the effects of the ion drive's propellant on the spacecraft and its local environment, and how well the drive is working as an engine. It's expected that when this mission is over, its "NSTAR" ion drive engine will be a proven and well understood piece of hardware which can be used by anyone for a primary means of interorbital propulsion.

Lunar and asteroidal propellants in electric propulsion

The Russian plasma thruster can use most any propellant in the thrust chamber, including oxygen.

However, some studies propose using oxygen as a source of propellant for ion drive as well. This is questionable. The main issue is the effects of ionized oxygen with the thruster materials (cathode, neutralizer, filament grids) of the ion drive engine. The thruster would need to either be made of different materials than today's ion drive engines and/or have easily replacable parts, neither problem being easily solved. A thruster design such as the German RIT10, which uses RF energy instead of a cathode to ionize the gas, may be the best present day design to start to consider for oxygen use. A second issue is that oxygen is not easily ionized, which means lower electrical efficiency. Thirdly, ion drive works best with heavier elements. Overall, oxygen does not look attractive for ion drive interorbital propulsion. (For satellite stationkeeping, the effects of ionized oxygen on the satellite would preclude its use.)

Xenon is the most popular propellant for ion drive today since it is a heavy gas (high atomic mass) that is easily ionized. Argon and mercury have also been used. Xenon and argon are inert gases which are not expected to be recoverable in useful quantities from the Moon and asteroids. The best candidate fuel for ion drive using lunar or asteroidal materials may be sodium, which is fairly abundant in some nonterrestrial materials, extractable without very much effort, easy to store and handle, and would work well with ion drive -- easily ionized, not damaging to the engine materials, and a relatively heavy element.

The Russian plasma thruster is different, and probably could use oxygen. It uses a separate, external cathode discharge and has no grid, so you can run just about anything you want through the thrust chamber, and use about 5% of, say, sodium, xenon, or argon in the cathode discharge chamber. JPL has done some preliminary work in the area and hasn't found any show-stoppers.

The next two sections are technical.

How ion drive works

Ion drive thrusters use an electric field to accelerate charged atoms or molecules (e.g., oxygen) to a high velocity as they are expelled out the thrust chamber, thus accelerating the spacecraft.

Ion thrusters generally use a cathode (a negatively charged grid similar to that found in a tv set) to generate a stream of electrons, which form an electric circuit with a positively charged ring - the anode. This stream of electrons is used to ionise the propellant. A small magnetic field is used to aid this process (electrons spiral around the magnetic field lines, increasing the chance of electron-atom collisions). The magnetic field may derive from either a permanent magnet or an electromagnet.

The ionised gas drifts towards an extraction grid system (two or three plates with many small holes in them, held at high voltage) where they are accelerated out of the thruster, so producing thrust. A neutraliser similar to the cathode is used to generate free electrons and balance the overall space charge in the outgoing beam so that the spacecraft doesn't charge itself up.

The electric power comes from a solar cell array. Of course, in orbital space, there is no air drag or weather forces, so the solar cell array doesn't need to be aerodynamic at all. Since the ion drive vehicle is relatively low thrust, the structural strength and mass can be low as well. For example, in a General Dynamics report: "The solar array performance was conservatively assumed for sizing purposes to be 150 watts/kilogram" based on a very conservative assumption of solar cells having only 7% efficiency. The assumed efficiency of ion drive at converting electrical energy into beam kinetic energy was 63%, though some ion thrusters today produce efficiencies between 70% and 90%.

Ion thrusters are modular. If you have more cargo or want to speed up your mission or slow it down to conserve fuel, then you can add or subtract thrusters and solar cell array units.

Ion drive engines have long lives, being subject to a much less stressful environment than chemical rocketry. Ion drive engines are also easier to work on, consisting of simple electrical components, in contrast to the high performance mechanical pumps bolted into chemical rocketry.

Ion drive was developed in laboratories in the 1960s, and there were the SERT1 and SERT2 experiments in space which proved that the drive would work in space for long periods of time and deliver significant propulsion to a spacecraft. When the space program shrunk due to poor political leadership after the Kennedy-Johnson era, ion drive was one area that saw research and development wane. However, some private communications satellites in geosynchronous orbit incorporated ion drive into their stationkeeping system once the satellite was delivered there by a chemical rocket and its solar cell array deployed to power the ion drive engine.

The NASA Lewis Research Center is developing a lower power version (about 700 Watts) of the DS1 NSTAR ion drive engine system. However, large scale space industrialization will use larger ion drive engines, or else many low power units together in a modular craft, the latter offering spacecraft security in case an engine or two fail.

In any case, ion drive is looking as if it will become a routine means of interorbital propulsion within the next 5 years for low cost scientific missions as well as some kinds of industrial missions.

Hall-Effect Stationary Plasma thrusters (Russian-French)

In the 1990s, an electrically powered propulsion technology used by the Russians was marketed outside of Russia, particularly in France. It was kept fairly confidential by those interests, but was referred to by various names, e.g., the Russian Stationary Plasma thrusters, the Hall-Effect thrusters, and the Russian Anode Layer thrusters.

Then came the announcement of the European SMART-1 probe to launch in October, 2002, to demonstrate this technology in a lunar orbiter probe. The home page for SMART-1 is sci.esa.int/smart-1/ but skip over to the subpage sci.esa.int/content/doc/10/2320_.htm for a good description.

The following explanation is a 1990s quote thanks to John Schilling (schillin@spock.usc.edu) of the University of Southern California's Aerospace Engineering:

"The thruster consists of a cylindrical chamber ~10cm in diameter with a central spike. Both spike and cylinder are made of ferrous material, and incorporate magnetic windings so as to create a transverse (radial) magnetic field across the exit. The walls are insulated, the base serves as an anode.

"External to the thruster is a separate, hollow, cathode. About 95% of the propellant (currently Xenon or Krypton) is metered into the thruster, and 5% to the cathode. In operation, a discharge is struck between cathode and anode. As electrons are much lighter than ions, one would expect most of the current to be carried by the electrons, but that same lightness makes it hard for them to cross the transverse field across the thruster exit. Some manage to trickle through anyway and keep the discharge running, but there is always a surplus of electrons hanging around outside the thrust chamber.

"This creates a strong electric field (~100 V/cm) which sucks out ions from the anode discharge region. Being fairly heavy, they don't so much notice the magnetic field, and with the field strength involved they end up moving at 15-25 kilometers per second. They pick up electrons from the accumulated surplus as they pass, not slowing for that any more than they did the magnetic field, and the resulting bulk-neutral plasma proceeds ballistically towards infinity.

"Oxygen would be a less effective propellant than Xenon, but it would probably work. You'd still need 5% of something non-oxidizing to run the cathode discharge on, but using 95% oxygen is not bad.

"The latest edition of Sutton's Rocket Propulsion Elements has a section on the Stationary Plasma thrusters, which is reasonably good."

MPD and other electric thrusters

Magnetoplasmadynamic (MPD) thrusters (aka magnetohydrodynamic (MHD) thrusters without the hydrogen association) and other kinds of electric propulsion techniques have been developed but not incorporated on spacecraft yet.

The page on the Southhampton University MPD research and development project describes the basics of the MPD thruster concept, gives a history of the Southhampton Univ. Dept. of Aeronautics and Astronautics MPD project, references, and lots of pictures of their MPD thruster research and development, thanks to kind web work by Alexander Fitzhugh.

Mass driver not considered seriously

PERMANENT gets a lot of e-mail suggesting we cover using a mass driver for interorbital transport. Basically, the mass driver has merit for lunar launch but not interorbital propulsion, compared to alternatives, in a near-term scenario. The moving parts entail considerable risk. Overall, it's too complicated. For more information on a mass driver for lunar launch, however, see the section on long-term propulsion where we have a section on the mass driver.

Passing through medium Earth orbit radiation belts

A significant problem with interorbital transfer between low Earth orbit and high Earth orbit is the Van Allen Belt of trapped high-energy particles (i.e., radiation) due to the Earth's magnetic field. The Van Allen Belt exists in a middle-level Earth orbit located below geosynchronous Earth orbit and above low Earth orbit. This radiation degrades the solar cells a little bit each time they pass through. This requires repair of the solar cells by an annealing process (heating and recrystallization) after a number of trips. Alternatively, interorbital vehicles could receive beamed power from a satellite stationed in high orbit, as discussed in the section on "Solar Power Satellites". A third option is to use nuclear electric vehicles.

The Van Allen Belt radiation affects only electric vehicles with solar cell arrays which are cycling between low Earth orbit and high Earth orbit. It will not affect cargos being hauled between two different high Earth orbits, or between the asteroids or lunar orbit and high Earth orbit.

Final words on electric propulsion

To date, the main drawback to electric vehicles has been the need to have an electric power plant. It is most attractive for fuel-efficient stationkeeping of satellites with sizeable electric power capacity, for deep space missions on a low budget which must get the most mileage out of their fuel, for satellites which already have sizeable power plant needs for other purposes (e.g., Russian military space based radar), and in the future as a reusable interorbital vehicle. Notably, DS1 is demonstrating low mass/high efficiency solar panels that will make a 3 kW power plant within reach of most spacecraft. DS1 is not a big spacecraft, and the whole scientific mission which includes studying an asteroid is being accomplished on a nearly record low budget. However, it is debatable if electric vehicles are the most economical means of propulsion for purely scientific missions which don't otherwise need large quantities of electric power. Nonetheless, electric vehicles are attractive for interorbital haulers in a large scale space industrialization scenario.

Links to other websites on electric propulsion

Hughes Aerospace Company produces the Xenon Ion Propulsion System (XIPS) which was first used on their previous generation 601 satellites with great success, and is heavily marketed in their current generation 702 satellites. You can also see a diagram of the XIPS. Hughes claims that their latest model of the XIPS is 13 times more efficient than conventional fuel propellant. The following is paraphrased from their web advertisement: XIPS needs only 5 kg of fuel per year for stationkeeping, a fraction of what bipropellant or arcjet systems consume. As a customer option, using XIPS as an upper stage "to help raise the spacecraft into final orbit" can save even more launch mass. Customers can apply the savings in launch mass to launch additional propellant to prolong satellite service life, or to increase the satellite mass to enhance its revenue-generating potential. Or, the savings in mass can be used to shift to a less expensive launch vehicle. They also suggest that using XIPS could allow some customers to add another payload or two to a given launch vehicle.

It's worth mentioning that Hughes' leadership in space communications launched an entire industry communications satellites. Hughes launched the Early Bird satellite more than 30 years ago, the world's first commercial communications satellite. Since 1965, HSC has launched more than 100 satellites, more than 40% of the commercial communications satellites currently in orbit -- and achieved a 99% on-orbit reliability record. Hughes has boldly led the way without sacrificing reliability, and this is the case with ion drive as well.

The Italian CENTROSPAZIO is conducting research in a variety of electric propulsion systems, including magnetoplasmadynamic (MPD) propulsion, arcjet propulsion, field emission electric propulsion (FEEP), and free electron drift propulsion (Hall thrusters). (Their vacuum facilities are also utilized for other research.)

If you know of any other sites on electric propulsion which are not listed here, please send a message to

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